Upper atmospheric observations at the Arecibo Observatory: Examples obtained using new capabilities

The Arecibo Observatory will soon complete a major instrumental upgrade whi ch will provide improved capabilities for observations of the upper atmosph ere. As in the past, Arecibo capabilities center on 430-MHz incoherent scat ter radar (ISR) measurements of the ionosphere made locally or in conjuncti on with other national and international incoherent scatter facilities; the upgrade will add the capability for simultaneous two beam incoherent scatt er observations and will extend plasma line measurements to +/-15 MHz. Aero nomical studies may also be performed using a 46.8-MHz coherent scatter rad ar, the feed of which is coaxial with the 430-MHz line feed, the newly refu rbished medium-frequency (MF) radar facility, and a new digital ionosonde. The original airglow laboratory continues to house the Fabry-Perot; interfe rometers, Ebert-Fastie spectrometer, and tilting filter photometers used fo r observations of mesospheric, thermospheric, and exospheric airglow, while a new laboratory provides a permanent home for the lidars used for resonan ce fluorescence observations of atomic metal layers in the mesopause region and for Doppler Rayleigh measurements of the upper stratosphere and lower mesosphere. Finally, the recently upgraded high-frequency, high-power trans mitter facility has performed active aeronomical and plasma physical studie s alone and in conjunction with other Arecibo and visitor-supplied instrume nts. Observations performed as the upgrade nears completion have already pr ovided a first. look at what lies ahead in areas as diverse and fundamental as the structure and dynamics of the turbopause, the properties of upper a tmospheric tides, the electrodynamics of the E and F regions, the dynamics of light ions in the topside, and the physics of plasma turbulence. Example s of these and other observations are presented, and the opportunities for future investigations are discussed.